Among mobile terminal apparatuses that mount protocol processing software such as TCP (Transmission Control Protocol)/IP (Internet Protocol), one that reduces CPU power consumption is known heretofore (e.g., Patent Document 1).
Patent Document 1 describes a technique of realizing low power consumption by predicting the time to transmit or receive the signal of each protocol and switching the CPU operation mode between a low power consumption operation mode and a normal mode.
Since mobile terminal apparatuses operate on a battery as a power supply, power consumption needs to be reduced to the lowest possible level. Especially processing in the CPU has a great influence on power consumption, and it is therefore necessary to avoid processing in the CPU as much as possible when the user does not operate the mobile terminal apparatus.
Furthermore, with increasingly sophisticated demands for telephone functions and security functions in recent years, mobile terminal apparatuses have come to feature software that performs a variety of types of protocol processing. For example, mobile terminal apparatuses may use PPP (Point-to-Point Protocol), IP (IPsec (Security Architecture for IP)), UDP (User Datagram Protocol), SIP (Session Initiation Protocol) and NAT (Network Address Translator) at the same time. Furthermore, software for performing protocol processing installed in a mobile terminal apparatus often sends/receives packets for maintaining session, on a regular basis, so as to maintain session even while the user performs no operation. For example, the “register request” message of SIP is a session-maintaining message. Moreover, PPP, IPsec or NAT can likewise use keep-alive messages for maintaining session.
FIG. 1 shows the timing of sending conventional session-maintaining messages. Since each message is sent in a separate cycle per protocol, the CPU operates at asynchronous timings between the protocols.    Patent Document 1: Japanese Patent Application Laid-Open No. 2004-192256